High contrast screen material

ABSTRACT

A high contrast projection or depixelating screen comprises a primary matrix of a first transparent material, bodies of a second transparent material of a different refractive index from the first material and, additionally, light absorbing or filtering bodies. In variants, the matrix is a light filtering material and incorporates discrete bodies of light transmitting material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a light (diffusing, optical screen materialsuch as may be used, for example, as a rear or front projection screenor as a depixelating screen for reducing the perception of individualpixels in a pixelated (e.g. LCD) display or, analogously, of tileindividual scan lines of a CRT display.

2. Description of the Prior Art

The formulation and processing of polymer materials to create diffuserssuitable as optical screen materials have been described in U.S. Pat.No. 2,287,556 (Land), U.S. Pat. No. 4,983,016 (Yamamoto), EP-A-0464499(Sumitomo) and EP-A-0843203 (Nashua). These specifications describesymmetrically and asymmetrically diffusing optical screen materialswhich may be created by extrusion or casting. Asymmetry in diffusion isimparted typically by stretching to create orientation. However all theabove materials appear “white” to a greater or lesser extent in ambientlighting conditions, that is without illumination from an imagingsystem. The whiteness is a function of light scattering by the dispersedparticles incorporated in the materials to render them light-diffusing.As a result of this whiteness, an image created optically in thediffusing material suffers from a lack of contrast. Contrast may berecovered or improved for example, by the addition of a polarising filmas described in the Sumitomo Japanese laid open Application Ser. No.5-113606 or by the use of a second layer of a tinted acrylic material.Although both these approaches can improve contrast, this is achievedwith a significant loss in brightness (gain), (as much as 50% where apolarising film is used).

It is an object of the present invention to provide a diffusing materialproviding improved contrast without a significant reduction inbrightness.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a highcontrast projection, depixelating, or the like screen, comprising aprimary matrix of a first, transparent material, a first quantity ofdiscrete bodies of a second transparent material of a refractive indexdifferent from the primary matrix material distributed in said primarymatrix and a second quantity of discrete light absorbing or filteringbodies distributed in said primary matrix.

According to another aspect of the invention, there is provided a methodof forming a screen according to the first-mentioned aspect, comprisingcompounding, in a molten or plastic state, a first light-transmittingthermoplastics matrix material with a second light-transmittingthermoplastics material insoluble in, and having a different refractiveindex from the first, and with a third thermoplastics material insolublein the first, said third material being light-absorbing or attenuating,the method further comprising extruding the resulting compound through aslot

Preferably, particularly where a high density of the discrete bodies inthe primary matrix is contemplated, the second and third plasticsmaterials are also mutually incompatible (i.e. mutually insoluble).

According to yet another aspect of the invention there is provided ahigh contrast projection, depixelating, or the like screen, comprisingdiscrete bodies of a first material and refractive index in a matrix ofa second material and refractive index, one said material beingtransparent and the other being light-filtering.

According to still another aspect of the invention there is provided Amethod of forming a screen, comprising forming a mixture comprising aplurality of discrete light-transmitting bodies in a fluid,light-filtering matrix or binder, forming the resulting mixture into athin layer or sheet, and causing or allowing at least said binder toset.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described below by reference to theaccompanying drawings, in which:

FIG. 1 is a schematic view, in cross section perpendicular to the planeof the sheet material, through a first form of light diffusing sheetmaterial embodying the invention,

FIG. 2 is a schematic view, in cross section perpendicular to the planeof the sheet material, through another form of light diffusing sheetmaterial embodying the invention,

FIG. 3 is a view similar to FIG. 2, illustrating a further embodiment,and

FIG. 4 is a view similar to FIGS. 2 and 3, illustrating a still furtherembodiment

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a high-contrast light-diffusing material embodyingthe invention comprises a transparent primary matrix 14 incorporating afirst quantity of discrete transparent bodies 12 embedded in the matrix14 and of a different, for example higher refractive index than thematrix 14, (these bodies being represented in FIG. 1 as unshadedcircles), and a second quantity of discrete light filtering orattenuating bodies 16 (represented in FIG. 1 by shaded circles), thebodies 12 and 16 being fully interspersed and distributed evenly but atrandom throughout the primary matrix 14. Each body 16 preferablycomprises a transparent matrix, (preferably of a different (e.g. higher)refractive index than the primary matrix) within which is distributeddiscrete light absorbing particles, such as particles of carbon-black.Alternatively, each body 16 may comprise a transparent materialincorporating a light filtering dye, such as a neutral (as regardscolour—i.e. grey) dye. In preferred embodiments of the invention, theprimary matrix material 14 and the bodies 12 and 16 are of plasticsmaterials, preferably thermoplastics, the matrix 14 defining a plasticssheet or film of a thickness of, typically 0.12 mm, with the meandiameter of the bodies 12, 16, being for example, of the order of 5 to10 microns (m⁻⁶). In the preferred embodiment, the ratio, by volume, ofthe fist bodies 12 to the primary matrix 14 is 15:85 whilst the ratio,by volume, of the second bodies 16 to the primary matrix is of the orderof 2:100. The percentage, by volume of pigment (carbon black) in thebodies 16 in the preferred embodiment is about 2.5%, so that the pigmentmakes up around 0.05% by volume of the screen as a whole.

It will be appreciated that the bodies 16 are, at 5 microns to 10microns diameter, significantly larger than (and additionally much moreregular in shape than (being generally spheroidal or ellipsoidal)), ticparticles of filler or pigment, (such as carbon black or silica)conventionally incorporated in plastics material. (The same alsoapplies, of course, to the bodies 12). It is contemplated that thelight-transmitting or absorbing, respectively, particles 12 or 16 neednot be of thermoplastics, but may be of other materials such asthermosetting resin, or glass (clear or pigmented) for example.

It will be appreciated that the structure described with reference toFIG. 1 may be achieved in several ways. For example, pre-formed solidbodies 12, 16 of the desired size may be mixed with a moltenthermoplastics polymer which is subsequently extruded, or extruded andblown, to form the sheet screen material, or respective quantities ofincompatible thermoplastics materials may be compounded in a plasticisedor molten state to break up the (molten) materials forming the minorproportions of the blend into discrete globules of the desired size,suspended in a continuous matrix formed by the molten major componentand the resulting material may be formed into a sheet or film by any ofa variety of methods known per se.

In the preferred embodiments, in which the bodies 12 and 14 are formedfrom thermoplastics incompatible with the thermoplastics matrix material14 and the compounded mixture, in a molten or plasticised state isformed into a thin film by a process in which a smaller diameter tubeformed by extrusion is inflated under internal pressure and whilst stillin plastically deformable state to a larger tube and the inflated tubeis drawn off (hauled off) mechanically (and, for example, rolled up) allby a process similar to that conventionally used in the manufacture ofplastic bags, the extrusion and hauling-off tend to stretch the matrix14 and the bodies 12, 16 in the direction of longitudinal extrusion,whilst the blowing tends also to stretch the matrix 14 and the bodies12, 16 circumferentially in the circumferential direction of the tube.Preferential elongation of the bodies 12, 16 in one direction in the“plane” of the sheet material renders the light diffusing properties ofthe material asymmetric, that is to say the material diffuses lightthrough a narrower angle in a plane perpendicular to the sheet materialand parallel with direction of elongation than in a plane perpendicularto the sheet material and to the direction of elongation. By controllingthe draw-off rate relative to the extrusion and inflation rates, thisasymmetry can be controlled or neutralised to produce a light diffusingmaterial having a controlled degree of, or no, diffusive asymmetry.Since, with the production method described, a screen material having nodiffusive asymmetry is one in which each body 12, 16 has, in principle,been stretched equally in all directions in the plane of the sheet, itwill be appreciated that in such a material the shapes of the bodies mayrange from oblate spheroids to circular lenticular or disc-like bodies.

Examples of manufacture of optically diffusing screens by the techniquedescribed above are described in more detail below:

EXAMPLES

In the following examples, an extrudable thermoplastics compound wasproduced by mixing the component polymers in a compounding extruderfitted with a cavity transfer mixer. The compound was then extruded intoa thin film using a conventional extrusion line incorporating filmblowing equipment. The process temperature was 180° C. The extrusion diehad a diameter of 180 mm with a (radial) die gap of 1.2 mm and theextruded material was blown to a diameter of either 400 mm or 490 mmenabling the production of two continuous films (by flattening the blowntube and slitting or trimming along opposite longitudinal edges of theflattened tube), of a width of approximately 24 inches (600 mm) or 30inches (720 mm) respectively. The results provided for Examples 1 and 2were taken from 30 inch width film, that is a bubble diameter of about480 mm. In the process, the bubble is collapsed to provide two filmseach representing half the circumference of the bubble. The reduction inthickness of 10:1 comes from the ratio of bubble to die diameter (about3:1) and haul-off rate. Bubble ratios in excess of 5:1 can be achieved.The intention, in the examples herein, was to produce a light-diffusingmaterial with a high angle of view and approximately symmetriclight-diffusing properties, i.e. diffusion of light substantiallyequally in all directions. The haul-off rate controls the symmetry ordegree of asymmetry in diffusion (for a given bubble ratio) whereas theextrusion rate that is, the rate at which the extruder pumps materialcontrols the angle of view (mainly by controlling the productthickness). For symmetric materials the haul-off rate and the extrusionrate should be very similar for the exemplified approx. 10:1 thicknessreduction.

Example 1

This example was intended as a comparison or “control” against which theperformance of a screen material embodying the invention could be fairlyassessed. Extrudable plastics material was compounded, blendingethylene/ethyl acrylate copolymer resin, LE 5861 (available fromBorealis/Distrupol) with polystyrene resin, N1910, (available fromVictor Plastics) in the ratio 85:15, and the resulting material extrudedthrough an annular die and blown as described above. The finished filmthickness after extrusion and blowing was 124 μm, Samples of this filmwere laminated to a polarising film, Polaroid type KE and separately toa tinted acrylic sheet, the neutral density of which was 0.3 and to asimilar acrylic sheet with a neutral density of 0.2. As theethylene/ethyl acrylatc copolymer resin and the polystyrene resin areincompatible (i.e. each is substantially insoluble in the other), thecompounding process resulted in a quasi emulsion or dispersion of minutedroplets of molten polystyrene resin in the molten acrylatc copolymerresin. Because of the difference in refractive index of the two resins,the film produced was light-diffusing.

Example 2

In this example in accordance with the present invention, thecompounding and extrusion process as described in Example 1 was repeatedexcept that the compound comprised, in addition to ethylene/ethylacrylate resin and clear polystyrene resin in the same proportions as inExample 1, 2% of pigmented polystyrene resin, type Lacqrene 163. Thefinished film thickness was 125 μm. (Polystyrene Lacqrene 163 black 1002is available from Elf Atochem and contains less than 3% carbon black).

The materials produced in accordance with Examples 1 and 2 were comparedin respect of angle of view, gain and contrast. The results aresummarised in Table 1 as follows:

TABLE 1 Angle of View Gain Contrast Material of Example 1 70 × 65 1.74 —(unlaminated) Material with polarised 71 × 64 0.86  0.33 layer Materialwith tinted acrylic 65 × 57 0.94 0.3 layer (neutral density 0.3)Material with tinted acrylic 65 × 57 1.18 0.2 layer (neutral density0.2) Material of Example 2 66 × 60 1.56 0.2

(The two figures in each entry in the Angle of View column in Table 1denote the angle of view in two orthogonal planes, the material being ineach case asymmetrically diffusing. It will be understood that thepolarised layer and tinted acrylic layer referred to were laminated tothe respective samples of the material of Example 1 in order toapproximate to the contrast-enhaneing effect of the pigmented bodies inthe material of Example 2).

Although the gain measurement for the material of Example 2 is slightlyreduced, as compared with the unlaminated material of Example 1, it issubstantially greater than for the other material of improved contrastillustrated in the table (and in which the improved contrast is achievedby lamination with either a tinted acrylic layer or a polariser layer).

Whist, for the purposes of the measurements illustrated in the abovetable, the material of Example 2 was not laminated to any othermaterial, it will be understood that, in any particular application,that extended, blown material may be laminated to a transparent orreflective substrate or superstrate.

The materials described may also be used with advantage in other areaswhere optically diffusive screens are utilised, for example, as layersto be incorporated on or in LCD displays, particularly pixelated LCDgraphic displays, for example to reduce the susceptibility of theindividual pixels.

Referring to FIG. 2, a high-contrast light-diffusing material embodyingthe invention, according to another aspect, comprises, arranged on asupporting substrate 110, a light-diffusing layer comprising discretetransparent bodies 112 embedded in a matrix 114 of a lower refractiveindex comprising a light-filtering material preferably with neutralfiltering characteristics (i.e. attenuating all wavelengths of coloursubstantially equally so as not to impart any colour “tinting”).

In the arrangement shown, the transparent bodies 112 are in the form ofspheres of substantially the same diameter as one another and thethickness of the matrix 114 corresponds substantially to the diameter ofsaid spheres, so that the spheres lie in a mono-layer resting on theupper surface of the substrate 110 and are just exposed at the freesurface of the matrix layer 114.

Where the layer 112 is transparent, a parallel beam of light directednormally onto the rear (lower) surface of the layer 10 will passtherethrough and the portions of the beam aligned with the transparentspheres 112 will be refracted thereby, (because the matrix 114 is of alower refractive index than the spheres 112) to exit from the layer 112,114 over a range of different directions, i.e. to be diffused. Eachsphere 112 thus acts as a tiny convex lens. It will be understood that,where each sphere extends precisely for the whole depth of the matrixlayer 114 and no further, any ray of light, perpendicular to the planeof the screen, passing through such a sphere 112 other than preciselyalong its vertical axis, will also have to pass through a certain amountof the matrix material 114 before reaching the sphere 112 and afterleaving the latter and before exiting from the diffusive layer and thuswill be, to some extent, attenuated by the matrix material 114. Clearlyif the matrix material 114 were entirely opaque, with the arrangementillustrated in FIG. 2, substantially no light could pass through theproduct. However, by judicious selection of the darkness of the filter114, it can be ensured that, for each sphere 112, the attenuation oflight for light entering the layer 112, 114, normally, (i.e.perpendicularly) within a radial distance from the central axis of thesphere which is only slightly less than the radius of the sphere itself,is significantly lower than for light passing normally through thematrix material 114 without passing through any sphere 112. Accordingly,light passing through the spheres 112 and thus subjected to the“scattering” or “diffusing” effect of the spheres, is very littleattenuated whilst light passing through the matrix material betweenadjacent spheres, is severely, preferably almost entirely, attenuated.It will be appreciated that, where the material is used as a rearprojection screen, for example, light which passes through the matrixmaterial without deviation will contribute nothing to image forming forany observer not viewing the screen along a line precisely perpendicularto tote latter. More importantly, however, the dark nature of the matrixsignificantly reduces, in such a scenario, the amount of ambient light,(and thus light without image content), which may be reflected towardsan observer, and thus minimises the loss of visual contrast experiencedby an observer in high ambient light conditions.

Conversely, FIG. 3 illustrates an arrangement which correspondssubstantially with that of FIG. 2 except that in the arrangement of FIG.3 it is the spheres 112′ which are opaque and the matrix material 114′which is transparent. In this case, the transparent matrix material 114′preferably has a refractive index higher than that of the spheres 112′so that light passing through the substrate 110 and entering the regionsbetween adjacent opaque spheres 112′ will either be totally reflected atthe boundaries between the spheres 112′ and the matrix 114′ or willtend, at any rate, to be refracted to pass only through minor portionsof the spheres 112′ and re-enter the transparent region, again enhancingthe proportion of the image-forming light scattered by the transparentregions as compared with light reflected from the other regions. Ineither case (FIG. 2 or FIG. 3) the effect is to enhance the screencontrast, as compared with a plain light-scattering Lambertian front orrear projection screen, thereby enhancing contrast in high ambient lightconditions.

It will be appreciated that, in practice, the desired effect may beachieved, at least to a major extent, even if the heights of the bodies112, 112′ relative to the upper surface of the substrate 110 do notcorrespond exactly to the depth of the matrix 114, 114′. Indeed, withthe arrangement described with reference to FIG. 2, the contrastafforded by the material may be significantly improved if the matrix 114is of a depth somewhat less than the heights of the bodies 112, perhapseven only half the heights of the spherical bodies 112. It will also beappreciated that the bodies 112 need not be spherical but may be of someother shape.

Furthermore, it will also be appreciated that the desired contrastenhancing effect may be achieved even if the matrix 114, 114′ is of asomewhat greater depth than the bodies 112, 112′ and, indeed, may beachieved where the bodies 112 are not arranged strictly in a mono-layerbut in a layer of several bodies deep, as illustrated in FIG. 4.

As with the embodiment of FIG. 1, the structure described with referenceto FIGS. 2 to 4 may be achieved in several ways. Thus, for example,pre-formed solid bodies 112 of transparent material may be mixed with atinted or dyed molten thermoplastics polymer which is spread on thesubstrate 110 in a thin layer in a molten state and allowed to harden orsolidify to form the material of FIG. 2 or, alternatively, tinted oropaque spheres 112′ may be mixed with a molten and transparentthermoplastics polymer which is likewise spread in a thin layer on thesubstrate 110 and allowed to harden or solidify, to form the material ofFIG. 3. Alternatively, of course, the initially fluid matrix materialmay be a monomer or other polymer precursor which is spread in a fluidstate onto the matrix 110 and thereafter irradiated or otherwise causedto polymerise or set In further variants of the method, mutuallyincompatible liquids, one being dyed or tinted, may be mixed together sothat small bodies or droplets of the one are dispersed within the otherto form an emulsion or quasi-emulsion which is spread onto the substrateafter which the materials are allowed or caused to harden, at least thematrix material is caused or allowed to harden.

Thus, a minor proportion of a polymer containing pigment or dye may bedispersed within the bulk of a transparent polymer material incompatiblewith the pigmented or dyed polymer, such that the pigment or dye remainswithin the first-mentioned polymer and is not dispersed in the bulk ofthe transparent material. As a result, when the resulting product isused as a projection screen, contrast is enhanced and gain substantiallyunaffected.

Whilst, in the above description with reference to FIGS. 2 to 4, it hasbeen assumed that the substrate 110 is transparent, it will beappreciated that the substrate 110 may be made reflective, for example,by being formed as a layer of reflective metal foil, where the materialis to be used as the front projection screen. Furthermore, of course,the substrate 110 may be dispensed with altogether.

If desired, the material described with reference to FIG. 1 may also bemade so thin that the bodies 12, 16, have diameters of the same order asthe thickness of the screen, so that the bodies 12, 16 effectively forma mono-layer. As with the embodiment of FIG. 1, die screen of any ofFIGS. 2 to 4 may be made by a process in which a blend of incompatiblethermoplastic polymers is formed into a thin film by forming a smallerdiameter tube by extrusion and inflating the tube under internalpressure and whilst still in plastically deformable state to a muchlarger tube, by a process similar to that conventionally used in themanufacture of plastic bags.

What is claimed is:
 1. A high contrast projection, depixelating or thelike screen comprising a primary matrix of a first, transparentmaterial, a first quantity of discrete bodies of a second transparentmaterial of a refractive index different from the primary matrixmaterial distributed in said primary matrix and a second quantity ofdiscrete light absorbing or filtering bodies distributed in said primarymatrix; wherein the bodies of said second quantity each comprise amatrix of a transparent material incorporating light absorbing or opaqueparticles; and wherein the transparent material or matrix material ofsaid second quantity of discrete bodies has a different refractive indexthan from said primary matrix.
 2. The screen according to claim 1wherein the bodies of said second quantity each comprise a transparentmaterial incorporating a light filtering dye.
 3. A high contrastprojection, depixelating or the like screen comprising a primary matrixof a first, transparent material, a first quantity of discrete bodies ofa second transparent material of a refractive index different from theprimary matrix material distributed in said primary matrix and a secondquantity of discrete light absorbing or filtering bodies distributed insaid primary matrix; wherein the bodies of said second quantity eachcomprise a matrix of a transparent material incorporating lightabsorbing or opaque particles; wherein the transparent material ormatrix material of said second quantity of discrete bodies has adifferent refractive index than from said primary matrix; and whereinthe materials of said first and second quantity have higher refractiveindices than said primary matrix.
 4. The screen according to claim 3wherein the bodies of said second quantity each comprise a transparentmaterial incorporating a light filtering dye.